The present invention relates to an aqueous cadmium oxide paste of high flowability for the vibration filling of foam-structure and fiber-structure electrode plaques.
Industrial cadmium electrodes for Ni/Cd storage cells are manufactured in diverse embodiments. The pocket plate contains, in perforated nickel-plated iron-sheet segments, so-called briquettes which are composed of mixtures of finely divided metallic cadmium, iron oxide and graphite. The finely divided cadmium is often produced by electrolytic deposition together with nickel from acidic solutions of cadmium and nickel salt. The electrolytic deposition can also be carried out directly on a carrier foil; the layer is normally subsequently compacted. Electrodes with plastic binding composed of CdO or finely divided Cd on metal gauzes are also known.
Sinter plaque electrodes, which are preferably used for high currents, vented and sealed cells, contain the active Cd material in the fine pores of the sintered plaque which are about 10 micrometers in size. These fine pores are impregnated by repeated soaking with a concentrated cadmium nitrate solution and precipitation of cadmium hydroxide in the pores by means of alkali hydroxide. The repeated soaking and precipitation is necessitated by the limited solubility of the cadmium salt. The process can be shortened by the electrochemical precipitation method (Kandler process) and results in the precipitation of cadmium hydroxide and cadmium. However, it is difficult to perform the Kandler process continuously because of the complicated chemical mechanism; it is therefore used only for producing high-grade electrodes.
Foam-structure and fiber-structure plaques have been used for about 15 years for electrode substrates and for supporting the active material. They are composed purely of metal or they contain in addition the structure-providing plastic or carbon basic body.
These plaques make possible a simple mechanical impregnation with a fluidized paste of active material. In contrast to the apertures of a gauze plate or grid plate, the pores of the plaque are small enough to hold the material introduced. However, compared with sintered plaques produced by powder metallurgy, the pores are larger, with the result that a suitably adjusted paste can penetrate almost completely and fill the cavities.
A suitable starting material for the paste is cadmium oxide, cadmium hydroxide or cadmium powder. At 4.8 g/cm.sup.3, the density of cadmium hydroxide is comparatively low. The achievable concentration of material in the electrode is consequently also low. Cadmium hydroxide pastes are therefore not very suitable. It is not possible to formulate a stable paste composed of cadmium powder alone since the metal particles show a tendency to settle. The density of cadmium oxide is 8.15 g/cm.sup.3. This high density makes it possible to produce cadmium electrodes with adequate cadmium concentration. Cadmium electrodes are used predominantly in nickel/cadmium cells.
In these nickel/cadmium cells, the necessary loading with active material and consequently the capacity are fitted to suit the positive nickel electrode capacity. A modern gastight nickel/cadmium satellite cell employing sintered electrodes (22nd IECEC, Philadelphia, August 1987, Paper 879076, Table 1) contains, for example, 12.4 g/dm.sup.2 cadmium material. With the specified plate thickness of 0.068 cm and 80% porosity, this corresponds to 2.28 g of cadmium per cm.sup.3 of pore volume. Converted to cadmium oxide, this corresponds to 2.60 g of cadmium oxide/cm.sup.3. It is not possible to achieve such filling densities in one operation with the cadmium oxide pastes hitherto known.
Cadmium oxide powder can be dispersed well in some selected organic liquids having a polar nature. For example, cyclohexanol, butyl glycollate or ethyl lactate are suitable. The dispersions employing these organic liquids can be produced, for example, in a blade mixer such as a Waring Blendor.
In this process, the loose, bulky agglomerates of the cadmium oxide primary particles are destroyed. Only by doing this are liquid dispersions containing up to 33% by volume of cadmium oxide fraction (2.69 g of cadmium oxide/cm.sup.3) obtained. These dispersions are, however, only of limited suitability for filling foam-structure and fiber-structure plaques. On the one hand, after a few hours, reactions are observed between cadmium oxide and fluid which alter the flowability (for example, in the case of butyl glycollate), and, on the other hand, demixing and settling occur (for example, in the case of cyclohexanol). The drying and the recovery of the fluid, and the disposal of the solvent vapors produced during drying present technical problems and are cost-consuming.
With water as fluid, a concentration of 21% by volume of cadmium oxide is achieved at best. In the course of few hours, such pastes thicken as a consequence of the reaction of cadmium oxide and water to form cadmium hydroxide. Japanese published specification (Japan Kokai Tokkyo Koho) 78-136,634 described an aqueous cadmium oxide paste containing 14% by volume of cadmium oxide in which the reaction of the cadmium oxide with the water is inhibited by adding sodium hydrogenphosphate, sodium pyrophosphate or sodium hexametaphosphate. This paste is as little suited to the achievement of high filling densities with a single filling as the cadmium oxide paste in 30% KOH as suspension fluid described in German Patent Specification 1,596,023. As is known, cadmium oxide reacts in aqueous KOH in a few minutes to form cadmium hydroxide in an exothermic reaction.
Accordingly, an object of the present invention is to provide an aqueous cadmium oxide paste which has such a high flowability that the pores of the foam-structure or fiber-structure electrode plaques can be competely filled with the aqueous cadmium oxide paste.
Another object of the present invention is to provide an aqueous cadmium oxide paste having a high flowability so that the pores of the foam-structure or fiber structure electrodes have such a high cadmium oxide content that the complete filling of the porous body can be achieved in one operation.